Stress reactivity may be an enduring individual characteristic, with important clinical and psychosocial associations. Both physiological and behavioral correlates of stress have been associated with infant temperament. Stress responses have generally been examined for inter- measure associations and longitudinal continuity. Yet, dissociations and discontinuities are prevalent. It is possible that one dimension of stress reactivity is the characteristic dissociation and discontinuity between measures and across time. Stress reactivity, which is sexually dimorphic in adults, may also differentially influence male and female infants providing the foundation for subtle sex differences in behavior. Our pilot data indicate that male and female fetuses produce different cortisol responses to the stress of labor. This suggests biologically based sex differences in stress reactivity. Crying is one the earliest and most significant infant behaviors, and is associated with different forms of stress. Infant sex differences in crying have been reported. This suggests that crying may be a sensitive and consequential behavioral stress response. The proposed study will examine the ontogeny of physiological stress reactivity, its inter-measure associations and continuity across time, and correlations with acoustic and non acoustic characteristics of crying during the first 4 months of life. 50 male and 50 female healthy, full term, vaginally delivered infants will be examined at birth, l month and 4 months of age. Heart rate (HR), plasma cortisol, epinephrine (EP), and norepinephrine (NE) response to labor will be measured. Salivary cortisol, EP and NE responses to a mildly noxious behavioral assessment and a mildly aversive stimulus will be measured at each age. Maternal assessment of infant temperament will be examined post-neonatally. Since there have been no studies of the association of acoustic cry characteristics and physiological stress reactivity, nor infant sex differences in stress reactivity, these studies are exploratory. Individual and sex differences in characteristic stress responses are expected within the first 4 months. A dimension of characteristic association and continuity of stress responses is anticipated. Replication of initial findings of sex differences in cortisol response to labor stress is foreseen. Sex differences in catecholamine response is expected within the first 4 months. Stress reactivity is also expected to be correlated with maternal temperament ratings. These studies will provide insight into the biological basis of individual and sex differences in stress reactivity, explore the behavioral expression of stress reactivity and further our understanding of temperament characteristics.